Invariant differential operators for non-compact Lie groups: Parabolic subalgebras

In the present paper, we start the systematic explicit construction of invariant differential operators by giving explicit description of one of the main ingredients - the cuspidal parabolic subalgebras. We explicate also the maximal parabolic subalgebras, since these are also important even when they are not cuspidal. Our approach is easily generalized to the supersymmetric and quantum group settings and is necessary for applications to string theory and integrable models.